It is well known that carbon fibers suitable for commercial applications may be produced from mesophase pitch. Carbon fibers derived from mesophase pitch have a high degree of molecular orientation and are light weight, strong, stiff, thermally and electrically conductive, as well as chemically and thermally inert. Mesophase-derived carbon fibers have been used as reinforcements in composites, have applications in the aerospace industry and are useful in quality sporting equipment. In contrast, carbon fibers produced from isotropic pitch exhibit little molecular orientation. As a result, they have relatively poor mechanical properties.
Mesophase pitch is not ordinarily available in existing hydrocarbon fractions, such as refining fractions, or in coal fractions, such as coal tars. However, methods are known for processing hydrocarbon fractions to obtain mesophase pitch. One well know method is to derive mesophase pitch from an isotropic pitch which contains mesogens. Isotropic pitches which contain mesogens are usually prepared by the treatment of aromatic feedstocks. Such treatment, which is well known in the art, may involve one or more heat soaking steps, with or without agitation, and with or without gas sparging or purging. Gas sparging may be carried out with an inert gas or with an oxidative gas, or with both types of operations. Numerous patents describe the preparation of isotropic pitch from aromatic containing feedstocks. Nonexhaustive but representative of such patents are: U.S. Pat. Nos. 4,283,269, heat soaking of fluxed pitch; Japanese Patent No. 65090/85, heating in the presence of an oxidizing gas; U.S. Pat. Nos. 4,464,248, catalytic heat soaking; 3,595,946 and 4,066,737, use of oxidative reactive material; and 4,474,617, use of oxidizing gas; and many others. Additionally, U.S. Pat. Nos. 4,184,942; 4,219,404; 4,363,715; 4,892,642 discuss the production and extraction of an isotropic pitch to obtain mesophase pitch.
In the past, mesophase pitch was commonly obtained by heat soaking a pitch feedstock to generate a mesogen containing isotropic pitch, followed by solvent fractionation to isolate the mesogens. In general, current solvent fractionation processes have the following steps:
(1) fluxing the isotropic pitch in a hot solvent, PA1 (2) separating flux insolubles by filtration, centrifugation, or other suitable means, PA1 (3) adding an anti-solvent to the clean flux filtrate (comix solvent) to precipitate the desired mesogens, PA1 (4) isolating the mesogens by washing and drying, and PA1 (5) fusing the mesogens to form mesophase pitch. PA1 "Pitch" as used herein means substances having the properties of pitches produced as by-products in various industrial production processes such as natural asphalt, petroleum pitches and heavy oil obtained as a by-product in a naphtha cracking industry, and pitches obtained from coal. PA1 "Petroleum pitch" means the residual carbonaceous material obtained from the catalytic and thermal cracking of petroleum distillates or residues. PA1 "Petroleum coke" means the solid infusible residue resulting from high temperature thermal treatment of petroleum pitch. PA1 "Isotropic pitch" means pitch comprising molecules which are not aligned in optically ordered liquid crystal. PA1 "Anisotropic pitch" or "mesophase pitch" means pitch comprising molecules having aromatic structures which through interaction are associated together to form optically ordered liquid crystals, which are either liquid or solid depending on temperature. PA1 "Mesogens" means molecules which when melted or fused form mesophase pitch. These molecules comprise a broad mixture of large aromatic molecules which arrange upon heating to form liquid crystals. An isotropic pitch can contain mesogens and these mesogens can be isolated by addition of an appropriate solvent. PA1 "Fibers" means filaments of lengths suitable for formation into useful articles. PA1 "Oriented Molecular Structure" means the alignment of mesophase domains in formed carbon-containing artifacts, which alignment corresponds to the axis of the artifact and provides structural properties to the artifact. PA1 "Oxidation/Stabilization" is the process of making a pitch artifact infusible or unmeltable by reacting the artifact with oxygen or an oxidizing agent. PA1 "Softening and Melting points" are determined by heating a sample at about 5.degree. C./minute on a hot stage microscope under an inert atmosphere. The softening point for a dried pitch is the first rounding of angular features of the pitch particles. The melting point for a dried pitch is that temperature at which the first observable flow of the softened pitch is seen. PA1 Clean isotropic feed pitch is a pitch which contains less than 500 ppm of mesophase insoluble components. Preferably the pitch will contain less the 250 ppm mesophase insoluble components. PA1 Mesophase insoluble components encompasses those compounds which will not dissolve in the mesophase pitch. Typically, mesophase insoluble components will include inorganic ash, coke and other compounds. PA1 Pitch oil is that portion of the pitch which boils at or below 525.degree. C. at atmospheric pressure.
This solvent fractionation procedure is well known in the art and is set forth in some detail in numerous patents. For example, U.S. Pat. No. 4,208,267 first disclosed that an isotropic pitch can generate a solvent insoluble fraction which becomes mesophase within minutes on heating to its melting point ("sintering"). This patent discloses an extraction process which utilizes a comix type solvent and the mesogens are collected as an insoluble residue.
U.S. Pat. No. 4,277,324, incorporated herein by reference, describes the foregoing solvent fractionation process and sets forth the conditions, procedures and solvents/anti-solvents which can be employed in solvent fractionation. Additionally, the '324 patent describes the fluxing of an isotropic pitch followed by filtering the flux mixture. The patent then describes the addition of an anti-solvent to precipitate the desired insoluble mesogens from the flux filtrate. Finally, U.S. Pat. No. 5,032,250, incorporated herein by reference, deals with supercritical liquid/liquid extraction of an isotropic pitch for directly producing a mesophase pitch. The solvent fractionation described by '250 occurs at elevated temperatures and pressures such that both the solubles and insolubles are in the liquid state.
It is desirable to provide an alternative process for obtaining mesophase pitch from isotropic pitch which produces a very clean mesophase. Further, it is desirable to provide a solvent fractionation process which does not involve the process steps, yield loss and waste generation associated with fluxing and filtering the isotropic pitch. Still further, it is desirable to provide a liquid/liquid extraction process that avoids solids handling and does not require the high temperature and pressure of supercritical fluid extraction. Finally, it is also desirable to control mesophase product hardness in this process without the high temperatures and pressures of supercritical fluid extraction.